This application relates to a semiconductor device and more particularly to a chip-scale semiconductor device and a method for manufacturing thereof.
Semiconductor devices such as power switches are a common part of electronic devices. Such devices are often packaged so that they may be practically incorporated into an electronic circuit. Due to the desire for smaller electronic devices there is a demand for electronic packages of smaller size. Chip scale packages thus have been developed which are the size of or nearly the size of a semiconductor device.
Also known are semiconductor device packages in which all electrodes are located on a common die surface and are connectable to corresponding conductive pads or traces on a support surface such as a printed circuit board.
The proper placement and attachment of a semiconductor package to a circuit board is an important step in manufacturing of electronic circuits. At the present time, such devices are first soldered down to the board and in a separate step a liquid epoxy is applied around the periphery of the device which is pulled into the space between the device bottom and the top of the board by capillary action to form an underfill to improve device temperature cycling capability. Often, such a process requires the device to be first soldered in an automated step before the underfill material is applied. Thus, if during the soldering process the device is placed less than perfectly there is a chance that the underfill material will not be deposited accurately and may unintentionally spill over to the other areas of the board. This of course is undesirable and may require the circuit board to be designed in such a way to accommodate spillage due to misplacement of the semiconductor package. For example, the area on the circuit board intended for the semiconductor package may be enlarged to accommodate the spillage of the underfill material. This solution of course requires the enlargement of the circuit board which is contrary to the desire for a smaller electronic device.
It would be desirable to provide a semiconductor package which can be accurately placed on a circuit board and attached thereto without the drawbacks of the conventionally known devices discussed above.
A semiconductor device according to the present invention is prepared to include the materials needed for board attach and underfill so that the user need only provide a thermal excursion to complete the attach process thus avoiding the problems encountered by prior art packages and attach methods.
In accordance with the invention a novel attach system, which can be made at wafer level, employs a silver loaded epoxy for the interconnects and a thermal epoxy which is disposed between the silver loaded interconnects for an underfill.
The preferred embodiment of the invention is a flip chip type semiconductor device which is processed at wafer level before singulation.
Specifically, a wafer having a plurality of identical die is first coated with a suitable photo-sensitive epoxy passivation layer. This layer is then processed to open windows over contacts of each one of the die. Then interconnects will be disposed on and connected to each contact of each die through a respective window.
According to an aspect of the present invention the interconnects are composed of a silver loaded epoxy having certain advantageous characteristics. Specifically, the silver loaded epoxy has a resin as a matrix material that is semi-solid and will be sticky to the touch at room temperature. The silver loaded epoxy may also include fillers which give the interconnects structural integrity both at room temperature and at higher temperatures.
To allow the silver loaded epoxy to be deposited on the contents of respective die it can be reduced using solvents. These solvents, when evaporated, leave the interconnects with a sticky exterior surface.
Next, a thermal epoxy is laid down across the surface of the wafer. The thermal epoxy, which will constitute the underfill layer, is solid at room temperature (staying solid to a slightly higher temperature than that used for the interconnects) and is reduced with a solvent. The thermal epoxy layer may be formed through a process of repeated deposition and evaporation to a thickness of slightly less than the height of the interconnects.
Once the thermal epoxy layer is formed the die are singulated by, for example, sawing thus forming a plurality of chip-scale semiconductor devices.
To achieve the intended purposes of the present invention, according to one aspect of the present invention, during a solder reflowing process such as that commonly used for SMT (surface mount) assembly, a semiconductor device according to the present invention is first subjected to a reflow process and then the thermal epoxy is cured to form the underfill structure. Specifically, when the device is first placed on the board the sticky interconnects of the device serve to keep the device in a proper place on the circuit board. As the temperature is elevated the interconnect resin starts to become mobile, but the fillers will prevent these interconnects from slumping at this stage. As the temperature increases still further, the thermal epoxy becomes mobile, and, having no structuring fillers, will become less viscous. Thus, the thermal epoxy flows readily and by capillary action is pulled toward the center of the semiconductor device.
In a preferred embodiment of the invention, once the temperature exceeds about 110xc2x0 C. the epoxy starts to cure. If necessary, however, this cure temperature can be increased. The epoxy resin and the hardener system of the thermal epoxy used in a semiconductor device according to the present invention are selected to be capable of fully curing within the solder reflow cycle.